It is common practice for decals to be used to temporarily show a registration mark while awaiting paperwork from the FAA Aircraft Registry. 14 C.F.R. 45.21 requires that registration marks must be painted on the aircraft or affixed by any other means ensuring a similar degree of permanence. 14 C.F.R. 45.21(d)(2) provides, however, that a registration mark may be affixed to an aircraft with readily removable material if it is bearing a temporary registration number. During our informal discussion with the FAA, we were unable to conclude that the exception may be applied to aircraft awaiting assignment of a special registration mark (form AC 8050-64). Thus, the FAA's preferred practice will require that registration marks be painted in this circumstance, rather than applied using one or more decals.